The 30-year-old has opened up about her eating disorder, and revealed it was her dad's divorce from Orianne Cevey in 2008 that was the catalyst for her illness.
"My life felt out of control. I couldn't handle the pain and confusion surrounding my dad's divorce," Lily told The Weekend Australian magazine. "I was having a hard time balancing being a teenager with pursuing two different grown-up careers - both of which I'd chosen myself but focused heavily on how I looked."
The "Tolkien" star - who was 19 at the time of the split - began starving herself, abusing diet pills and laxatives and exercising obsessively, which continued into her twenties.
Lily, whose mother is the musician's second wife Jill Tavelman, has previously discussed her turbulent relationship with her famous father, and admitted it was his divorce from Orianne that was the beginning of their "terrible disconnect" that lead to them being estranged.
"Many of my deepest insecurities stem from these issues with my dad," the "Mirror Mirror" star wrote in her 2017 memoir, "Unfiltered: No Shame, No Regrets, Just Me".
Lily gorged on "every type of junk food possible," later forcing herself to throw up, causing her hair to fall out and her nails to become brittle.
"My throat burnt and my oesophagus ached. My period stopped for a couple of years. I was terrified I had ruined my chances of having kids. I was convinced that I had f**ked myself up beyond repair," she shared.
Phil and Orianne later reconciled in 2016, and the pair now live in Miami, Florida, with their two sons, Nicholas and Matthew.
The former Genesis musician also shares son Simon Collins and daughter Joely Collins with ex-wife Andrea Bertorelli.